An electric power steering apparatus that performs a control action such that an assisting torque for steering steerable road wheels of a vehicle is produced by an electric motor in dependence on the steering of a steering wheel is conventionally known. As the control action performed by the electric power steering apparatus is mainly based on the magnitude of the steering angle in relation to the steering wheel neutral position (straight-ahead condition), a proper assisting steering torque cannot be obtained when the steering angle sensor for detecting the steering angle has become faulty or otherwise unable to operate normally. It was previously proposed to stop the drive of the electric motor altogether upon occurrence of such an abnormal condition (See patent document 1 and 2, for instance).
[Patent document 1, Japan laid open patent 2002-104218]
[Patent document 2, Japan laid open patent 2005-67262]
When the drive of the electric motor is stopped due to an occurrence of a failure, the returning force to return the steering wheel to the neutral position will have to depend on the self-aligning torque that is produced by the suspension geometry of the vehicle, and acts as a returning force to restore the steerable road wheels to the straight-ahead condition.
In an electric steering power apparatus, the torque transmission mechanism such as a gearbox is interposed between the steering wheel and the knuckle arms of the steerable road wheels, and the friction such as that of the gearbox affects the returning force to restore the straight-ahead condition. In case of a relatively large steering angle, the self-aligning torque is significantly larger than the friction, but when the angle is within a small range around the steering wheel neutral position, the self-aligning torque may become smaller than the friction.
In the normal case, the control action applied to the electric motor to create a returning force to return the steering wheel to its neutral position is performed until the detected value of the steering angle by the steering angle sensor becomes equal to a value corresponding to the neutral position of the steering wheel (zero degree steering angle). However, in the case of a failure of the steering angle sensor, the control action based on the steering angle detection value is not possible, and the only available returning force will be the self-aligning torque such that the steering wheel may not be able to return to its neutral position when there is a region of a relatively high friction.